1. Field of the Invention
The present invention relates to an optical scanning device and an image forming apparatus using the optical scanning device, and more particularly to the optical scanning device in which an amount of change in a relative scanning position of each scanning optical system, which is caused by a temperature fluctuation, is decreased.
2. Discussion of the Background
An image forming apparatus, in which a plurality of photoconductive elements are scanned by single optical scanning device is commonly known. The image forming apparatus includes, for example, a color printer having a plurality of photoconductive drums, a color copier having a plurality of photoconductive drums, a high speed laser printer, a digital copier and so forth. In the above-described image forming apparatuses, a high speed print of a color image is performed because individual photoconductive elements corresponding to each color are provided. In addition, the number of parts used including a deflector and a consumption of an electric power are decreased because a single optical scanning device is commonly used for scanning the plurality of photoconductive elements.
In the above type of image forming apparatus, a lens made of a resin, which has a positive power only in a sub-scanning direction, is used as an imaging lens to form a linear image in the vicinity of a deflecting surface of a deflector. Hereinafter, a direction in which beam light emitted from a light source is deflected by the deflector is referred to as a main scanning direction. A direction perpendicular to the main scanning direction and in which a transfer sheet is conveyed is referred to as a sub-scanning direction. As the imaging lens, a lens made of a resin is commonly used instead of a lens made of a glass to reduce costs for a material itself and processing of the material.
However, because the resin lens has a high linear expansion coefficient, an optical axis of the lens changes in a sub-scanning direction when a surrounding temperature fluctuates. Thus, a scanning position on a surface of a photoconductive element changes in the sub-scanning direction. If a direction of change of the scanning position in the sub-scanning direction is different on each photoconductive element, a color shift is created, resulting in a degradation of a produced image.
In Japanese Patent Laid-Open Publication No. 10-26732, an optical scanning device and an image forming apparatus using the optical scanning device in which a hybrid cylinder lens, which is integrally produced with a resin lens having a negative power in the sub-scanning direction and a glass lens having a positive power in the sub-scanning direction are disclosed. The hybrid cylinder lens is employed to reduce an image surface bulge fluctuation caused by the temperature fluctuation. However, the image surface bulge fluctuation in the sub-scanning direction is caused by the temperature fluctuation even if the hybrid cylinder lens is employed because a resin has a linear expansion coefficient higher than that of a glass.